1. Field of the Invention
This Invention relates to an image forming apparatus, and particularly to an apparatus for forming images by a plurality of image forming portions.
2. Related Background Art
There is known an apparatus for forming images of different colors by a plurality of Image forming units each having a laser scanner, a photosensitive drum, etc., and superimposing and transferring them onto a recording sheet to thereby obtain a color image.
In the apparatus of this kind, to accurately superimpose and transfer the images formed by the respective image forming units onto a recording sheet, the image formation starting positions in the main scanning direction and the sub-scanning direction of each image on each photosensitive drum must be adjusted accurately. For this purpose, there is known the technique of transferring a register mark for the correction of misregister onto a transferring belt, and detecting and correcting the misregister between the images of respective colors on the basis of the result of the reading of the register mark.
Regarding the misregister in the main scanning direction, the writing-out timing of each image is adjusted in each image forming unit on the basis of the horizontal sync signal of each light beam, whereby the misregister between the images can be corrected even if the surface phases of respective scanner motors are not coincident with one another.
Also, regarding the misregister in the sub-scanning direction, rotation reference signals to respective scanner motor controlling portions are made to have the same cycle period and the rotating operations of the respective scanner motors are controlled, whereby the rotational speeds of the scanner motors can be made coincident with one another. Also, by a phase difference being given between the rotation reference signals, the surface phase of each scanner motor can be kept at a predetermined phase. By using these techniques, the writing-out timing of less than one (1) line in the sub-scanning direction can be corrected to thereby correct color misregister.
Generally, an image forming apparatus using the electrophotographic process has a fixing device for passing a printing sheet bearing toner images thereon through the pressure contact portion between a pair of fixing rollers, and mixing and fixing the superimposed toner images of a plurality of colors by heat and pressure.
For example, when printing is to be effected on a transparent sheet for projection (OHT), to improve the transmitting property thereof, it becomes important for toner images to be sufficiently melted and mixed with one another, and for the surface of the sheet to be smooth. For this purpose, it becomes necessary to reduce the fixing speed and supply sufficient heat to the toners.
As means for realizing a printing operation in which the fixing speed is reduced, there is a method of reducing the speed of not only the charging, developing and fixing processes directly concerned with the speed in the sub-scanning direction (the sheet conveying direction) but also the exposing process, with the fact that the charging, exposing and development of a pertinent page are effected in parallel with one another taken into account. For that purpose, when in the in-line type apparatus having the plurality of image forming units as previously described, the fixing speed is 1/n, it would occur to mind to effect exposure based on image data only once per n times of scanning of the scanner motor.
However, when as previously described, the exposing process based on image data is effected only once per n times of scanning of the scanner motor, even if an attempt is made to control the surface phase of the scanner motor to thereby correct the misregister in the sub-scanning direction, the range within which a phase difference can be given to the surface phase of the scanner motor to thereby correct the misregister is limited to 1/n line or less.
This state will now be described with reference to FIG. 13 of the accompanying drawings.
FIG. 13 shows the state of images formed when exposure based on image data was effected only once per two times of scanning of the scanner motor.
In FIG. 13, 1301K, 1305K and 1309K designate black images formed by one time of exposure per two times of scanning of the scanner motor, and 1301Y, 1305Y and 1309Y denote yellow images formed by one time of exposure per two times of the scanner motor. 1301K, 1305K, 1309K and 1301Y, 1305Y, 1309Y deviate by 1311 in the sub-scanning direction from each other. Also, broken lines indicated by 1303K, 1303Y, 1307K and 1307Y indicate the states of black and yellow images which are thinned by one time of exposure per two times of scanning of the scanner motor and are actually not formed.
When as shown in FIG. 13, the deviation 1311 is one time of scanning of the scanner motor, i.e., one (1) line or less, the phase of the scanner motor is quickened in conformity with the deviation 1311 when the yellow images 1301Y, 1305Y and 1309Y are formed with the black images as the reference, whereby misregister can be corrected.
On the other hand, when as shown in FIG. 14 of the accompanying drawings, the misregister amount in the sub-scanning direction between the black image and the yellow image exceeds one time of scanning of the scanner motor, i.e., one (1) line, as indicated by 1313, the amount which can be corrected with the phase of the scanner motor changed during the formation of the yellow images 1301Y, 1305Y and 1309Y is 1315 indicated in FIG. 14, and the difference 1317 between the deviation 1313 and the deviation 1315, i.e., misregister of one (1) line corresponding to one time of scanning of the scanner motor, cannot be corrected.
That is, when between the actually formed images 1301K, 1305K, 1309K and 1301Y, 1305Y, 1309Y, there is misregister exceeding xc2xd line, misregister of xc2xd line will remain even if the rotational phase of the scanner motor is corrected.
It is an object of the present invention to solve the above-noted problems.
It is another object of the present invention to obtain images of high definition.
It is still another object of the present invention to correct the misregister of images accurately and effect image formation.
The image forming apparatus of the present invention is provided with:
a plurality of image forming means juxtaposed with one another and each having an image bearing member, light beam generating means for generating a light beam modulated in conformity with an image signal, a rotary polygon mirror for deflecting and scanning the light beam generated by the light beam generating means and writing an image onto the image bearing member, and light beam detecting means for detecting the light beam deflected and scanned by the rotary polygon mirror at a predetermined position on a scanning route;
sync signal generating means for generating a horizontal sync signal based on a detection signal from the light beam detecting means;
mask means for masking and outputting the is horizontal sync signal;
image signal outputting means for outputting the image signal to the light beam generating means in the plurality of image forming means in response to the horizontal sync signal outputted from the mask means;
a plurality of rotation controlling means for rotatively driving the rotary polygon mirrors in the plurality of image forming means based on light beam detection signals from the light beam detecting means and a rotation reference signal:
reference signal generating means for generating the rotation reference signal to the plurality of rotation controlling means; and
control means for changing the phase of the rotation reference signal and the mask timing by the mask means based on the misregister amount among images formed by the plurality of image forming means to thereby correct the misregister amount among the images.
Other objects, constructions and effects of the present invention will become apparent from the following detailed description and the accompanying drawings.